This invention relates to dampers, such as shock absorbers or damping struts, for vehicles More particularly, this invention relates to shock absorbers and damping struts in which damping force provided by fluid flow is adjustable either manually or automatically in response to vehicle speed, load, acceleration, or road conditions.
In one typical suspension system for automobiles, shock forces experienced by the suspension system are "absorbed" by a spring and shock absorber combination. The shock absorber consists of a rod and piston mounted within a cylinder, and the spring urges rod to extend outwardly from the outer cylinder Vibrational shock energy within the suspension system is "absorbed" and stored by the compression and extension of the spring. That energy is, in turn, dissipated by the shock absorber or strut during the damping action that results from a valved piston reciprocating within the cylinder containing hydraulic fluid. The damping action converts the vibrational energy to heat which the cylinder wall conducts and transfers to the surrounding atmosphere.
One typical shock absorber has an oil cavity in the cylinder on each side of the rod piston within the cylinder. Compression of the shock absorber (as the spring and piston rod are compressed in response to contact of the wheel with the road) forces oil from one cavity into the other, and rebounding is subsequently damped by the resistance of oil flow in the "reverse direction as the piston is urged toward extension by the rebounding force of the spring. The valving within the piston and between the cavities thus provides the appropriate resistance to fluid flow between the two cavities.
For example, various types of check valves and variable flow restrictions have been developed to allow fluid flow in compression (from the cavity beneath the piston through or around the piston head and into the outer cylinder reservoir cavity) more freely, i.e., with less resistance, than in extension (from the outer reservoir cavity, and from the cavity surrounding the rod into the cavity beneath the piston). Such check valves thus provide greater damping when desired, during extension (rebound), rather than during compression shock absorption, when the shock absorber should compress relatively freely to minimize energy transfer to vehicle occupants.
Improvements on this basic arrangement have provided varying damping characteristics for varying road conditions or vehicle speed, load, or acceleration. As a vehicle goes faster, for example, the damping should often be increased to counteract the increased forces that tend to reduce contact between the tire and the surface of the road.
One such variable damping shock absorber is shown in U.S. Pat. No. 4,527,676 issued Jul. 9, 1985 to Emura et al. The Emura patent discloses a shock absorber with variable damping accomplished by a motor rotating a four-way check valve in the shock. The motor is either (1) a step motor with four steps, one for each of the possible orientations of a four-way check valve, or (2) a feedback-controlled motor, with four arcuate electrical leads contacted by brushes to determine the exact position of the motor drive. Both types of motors are relatively complicated and expensive, as is the four-way valving associated with each.
For example, the Emura device requires a four-wire harness. This results in an expensive and complicated vehicle harness.
Another problem with the Emura device is its relatively complicated and expensive motor mechanism In the embodiment utilizing a four-way step motor, such motors are inherently complicated and expensive because they must be adapted to stop at each of four points, with appropriate electronics to maintain control of the position of the step motor at each of its four stop positions.
In Emura's embodiment utilizing a feedback-controlled motor, the motor provides four-way stepping through a feedback control circuit which receives the feedback from mechanical wipers brushing over electrical contacts. These wipers and contacts wear and corrode over time, causing loss of electrical contact. Emura's variable damping mechanism can thus completely fail long before likely breakdown from any of the remaining components.